This scene is the hinge of the entire play — everything before it builds toward it, and everything after is its consequence. Track the chain of cause and effect closely:

1. Tybalt challenges Romeo, who refuses to fight (for reasons Tybalt doesn’t know).
2. Mercutio, seeing Romeo’s refusal as “calm, dishonourable submission,” fights Tybalt in his place.
3. Romeo tries to separate them — and his intervention is what allows Tybalt to wound Mercutio fatally.
4. Mercutio’s dying line, “A plague o’ both your houses,” curses both families and reframes the feud as the true villain of the play.
5. Romeo, now enraged, kills Tybalt — reversing his earlier restraint entirely.

Notice that Romeo’s fatal action here is caused directly by his attempt to keep the peace — a pattern worth remembering when you reach the Themes module’s discussion of fate versus choice.